Semiconductor laser structures include a p-n junction across which current flows (the conventional current from p to n) and an "active layer" in which electrons and holes combine with the production of photons by stimulated emission. The active layer has to relate suitably in band gap and refractive index to the other semiconductor regions of the structure in order to achieve a suitable degree of "confinement" of these processes to the active layer. The layers of material to either side of the active layer and in contact with the opposite faces of the active layer are known as "confinement layers".
A major field of application of semiconductor optical devices is in optical fibre communications systems. Silica optical fibres as produced in recent years have loss minima at 1.3 .mu.m and 1.55 .mu.m approximately, the latter minimum being the deeper. Accordingly, there is an especial need for devices operating in the range from 1.1 to 1.65 .mu.m, especially from 1.3 to 1.6 .mu.m. (These wavelengths, like all the wavelengths herein except where the context indicates otherwise, are in vacuo wavelengths.) Semiconductor lasers operating in this region of the infrared usually comprise regions of indium phosphide and of quaternary materials indium gallium arsenide phosphides (In.sub.x Ga.sub.1-x As.sub.y P.sub.1-y). By suitable choices of x and y it is possible to lattice-match the various regions while varying the band gaps of the materials. (Band gaps can be determined experimentally by, for example, photoluminescence.) Additionally, both indium phosphide and the quaternary materials can be doped to be p- or n-type as desired.
Semiconductor lasers comprising regions of gallium aluminium arsenide and gallium arsenide are also used for communications purposes. These operate near to 0.9 .mu.m.
Ridge waveguide lasers have been previously described, for example, in the following publications of Kaminow and his coworkers: Electronics Letters, 1979, volume 15, pages 763-765; Electronics Letters, 1981, volume 17, pages 318-320; Electronics Letters, 1983, volume 19, pages 877 to 879. The ridge of the ridge waveguide laser is present to afford transverse optical beam confinement. However, we believe that in practice the particular structure favored by Kaminow and his coworkers, involving electrical contact with the ridge through a window in a dielectric layer covering the ridge and the valleys to either side of the ridge, makes for low yields of devices having narrower ridges (i.e. of those devices having better transverse confinement performance in principle).
In West German Offenlegungsschrift No. 2422287, of Siemens AG, a ridge waveguide laser is described wherein electrical contact with the ridge is made through a window in semiconductor material made resistive by bombardment with protons. However, again we believe that in practice this structure makes for low yields of devices having narrower ridges.
An advantage of ridge waveguide devices is that they are capable of high modulation speeds (see in particular the 1983 paper cited above), and this is of course desirable for communications purposes, permitting a higher data transmission rate other things being equal.
Longitudinal mode control is another important factor in semiconductor laser design. In general, a laser will end to operate in several longitudinal modes corresponding to differing emission wavelengths, whereas both for telecommunications and for other purposes it is often desirable that the laser power should be concentrated into a very narrow wavelength range. In the case of telecommunications systems with silica fibres, logitudinal mode control is especially important for operation near 1.55 .mu.m where the dispersion in the fibre is usually much greater than near 1.3 .mu.m. Moreover, Fabry-Perot lasers are in practice difficult to incorporate in integrated optics structures.
Longitudinal mode control to avoid such problems can be achieved by means of a diffraction grating. One laser structure incorporating a diffraction grating is known as the distributed feedback (DFB) laser (see G. H. B. Thompson, Semiconductor Lasers, Wiley, 1980). In these, the p-n junction across which the current flows lies beneath or above the diffraction grating (in contrast to "Bragg" lasers where the said junction is not beneath the grating). (Here, and throughout this specification, terms such as "beneath", "above", "up", "elevated", etc. are to be taken to indicate merely a reference direction and not the actual orientation of a device in space.)
DFB lasers employing some variant of the buried heterostructure have been described, e.g. by Utaka et al, Electronics Letters, 1981, volume 17, pages 961 to 963; by Itaya et al, 1982, volume 18, pages 1006 to 1008; and by Kitamura et al, Electronics Letters, 1983, volume 19, pages 840 to 841. The buried heterostructure affords transverse optical confinement and gives operation at low threshold currents. Stable operation in a single longitudinal mode up to output powers of 38 mW has been observed. However, the production of satisfactory buried heterostructures involves a complex series of precise growth and etching steps, which makes for low yields of good devices. Moreover, we believe that the various reverse-biased current-blocking layers in such structures are associated with parasitic capacitances that limit the modulation rate.